1. Field of the Invention
This invention relates to a display device used for a monitor for a computer and a television, and particularly a display device having three terminals of an anode, a cathode, and a gate, in which the cathode and the gate are connected in a matrix manner, and a drive control method therefor.
2. Description of the Related Art
Recently, a flat-typed display device has been noticed which uses an electron emission element.
The electron emission element is grouped as a hot cathode type and a cold cathode type. The cold cathode type is mainly used for the display panel of the flat-typed display device. As such a cold cathode type, a field emission type (hereinafter called “FE-type”), a metal/insulating layer/metal (hereinafter called “MIM-type”), a surface conducting type (hereinafter called “SC-type”) and so on are known.
A famous FE-typed example is disclosed in following document; “C. A. Spindt. “Physical properties of thin-film field emission cathodes with molybdenium cones”, J. Appl. Phys., 47,5248 (1976). A known MIM-typed example is disclosed in following document; “C. A. Mead, Operation of Tunnel-Emission Devices”, J. Appl. Phys., 32,646 (1951). A known SC-typed example is disclosed in following document; “M. I. Elinson, Radio Eng. Electron Phys., 10,1290(1965).
A display panel using any of the electron emission elements as an electrical source is realized by a constitution having a substrate where cathodes and gates are formed in XY-matrix manners, and anodes having a fluorescent member opposing to the substrate are arranged, thereby electrons emitted from an electron emitter of the cathode are irradiated onto the fluorescent member at the anode side resulting in allowing the fluorescent member to be emitted.
As such an electron emitter, an electron emitter made of a carbon group material or a fibrous material having a smaller working function for electron emission and a lower threshold voltage is being noticed. Examples of usage of these electron emission elements are disclosed in before-mentioned Patent documents 1 to 3.
In any of these documents, it is disclosed that fluerene, diamond, diamond-like carbon (DLC), carbon nano tube (CNT), fibrous carbon, or the like is used as an electron emitter.
Thus in case of three terminals type of an electron emission device having a lower threshold voltage, a voltage is not applied between cathode and gate. Merely a normal high voltage (called “anode potential”) has only to be applied between anode and cathode, resulting in that electrons are emitted from a electrical releasing member being provided at the cathode under field electron emission phenomenon. Accordingly, when emitted, electron emission is performed without applying a voltage between cathode and gate. On the other hand, when not emitted, electron emission is refrained by applying a stop voltage between cathode-gate. An operation like this will now be defined as “normally-on-type” operation.
Hereinafter, a single normally-on-type electron emission device using an electrical emitting member made of carbon fiber will be picked up and explained as an example.
FIG. 12A and FIG. 12B are schematic views showing a voltage distribution of a single electrical releasing member. FIG. 12A shows a voltage distribution in a drive state in which electron emission is performed. FIG. 12B shows that in a stop state in which electron emission is stopped.
In a state shown in FIG. 12A, an electrical field which is larger than a threshold value electrical field that electron emission will be started in an electron emitter 5 on a cathode 2, is generated only using a voltage between the cathode 2 and an anode 6. Namely, FIG. 12A shows a drive state in which an electron emission occurs. Now, such a state is called “normally-on-state”.
For example, if a threshold electrical field of the electron emission member 5 is defined by 3 V/μm, if the anode 6 is provided at a position of 2 mm away from the cathode 2, and if the cathode voltage is applied by 0 V and the anode potential between the cathode 2 and the anode 6 is applied by 6 kV, the electron emission will be started.
In order to establish a preferable normally-on-state, higher anode potential may be applied. The anode potential may be decided depending on an electrical field intensity by which a necessary current density can be obtained by a voltage-current characteristic of the electrical emission device.
For example, if the necessary current density is obtained at the electrical field intensity having 5 V/μm and if the anode 6 is provided at a position of 2 mm away from the cathode 2, 10 kV has only to be applied as the anode potential.
FIG. 12A shows a mode of equipotential surfaces at this time. In FIG. 12A, there exist the substantially equipotential surfaces between the anode 6 and the electron emitter 5, and the electrical field intensity near the electrical emitter 5 becomes about 5 V/μm, resulting in that electron emission will occur.
Further, a voltage to be applied between the cathode 2 and the gate 4 for electron emission should be a voltage except for a voltage which prevents the electron emission by the anode potential. Namely, any voltage except for 0 V can be also available. More preferably, the voltage may be a voltage which provides no uncomfortable affection to the electrical field intensity. In the above-mentioned normally-on-state, an example is shown in which the voltage is set to be 0 V.
On the other hand, in the state shown in FIG. 12B, if a negative potential is supplied to a gate 4 with respect to a cathode 2, an electrical field intensity to be received from the anode 6 becomes smaller near an electrical emitting member 5 i.e., the electrical field intensity becomes intensity of a necessary threshold value electrical field or smaller, resulting in that the electron emission is stopped. The voltage between the cathode 2 and the gate 4 at this time is called stop voltage”.
The equipotential surface when a stop voltage is applied between the cathode 2 and the gate 4 will be explained as follows. As shown in FIG. 12B, voltage of each of The cathode 2 and the electrical emitting member 5 is 0.0 V while potential of the gate 4 becomes negative, so that a distance between the equivalent voltage surfaces near the electrical emitting member 5 becomes wider, resulting in that it will be found that the electrical field becomes smaller.
The stop voltage to be applied between the cathode 2 and the gate 4 at this time will be decided as follows. First, necessary electrical field intensity is decided by a threshold value electrical field of the electrical emitting member 5 and a necessary electrical field intensity by the anode potential in a normally-on-state, and thereafter the stop voltage is properly decided by design of a dimension of the electrical emitting member 5, a distance between a cathode and a gate, a dimension of a gate and so on.
As above, in the normally-on-typed electrical emitting member, electron emission is performed merely by applying a voltage between a cathode and an anode, and electron emission is controlled by stopping electron emission by applying a stop voltage between the cathode and a gate. As a result, there is no necessity that a voltage between cathode and gate becomes equal or more than a threshold value, resulting in that stable drive control becomes possible at lower voltage.
[Patent Document 1]
Japanese Patent Laid-open No.2000-251783 gazette
[Patent Document 2]
Japanese Patent Laid-open No.2000-268706 gazette
[Patent Document 3]
Japanese Patent Laid-open No.2002-100279 gazette
Then, it has been considered that such a normally-on-typed electron emission device is applied to an XY-matrix-typed and flat-typed display device. In a case where such a flat-typed display device, a voltage is applied between cathode and gate, which provides an electrical field intensity having a value being equal to or more than a threshold value, and when a stop voltage is not applied between cathode and gate, all-white-display will be performed at a most luminosity over all the surface of the display screen.
Accordingly, when such a flat-typed display device is used as a television or a computer monitor, if the all-white-display is performed even for a short time, a user often incorrectly recognizes that the device might be out of order and/or feels uncomfortable.
Further, like the above-mentioned fibrous material or nano constitutional body, when an electron emitter having a relatively low threshold value voltage for emitting electrons, if a voltage between a cathode and a gate lies under no-control state, a condition is continuously kept that electron emission is apt to occur owing to the voltage between the cathode and an anode, resulting in that though all-white display might not be established at a most luminosity, there might be a fear that light-emission occurs owing to unintentional electron emission.
Above all, all-white-display and unintentional electron emission are apt to occur in a case where an anode potential is transited from a stop state to a supply state depending on occurrence of a display start signal, for example when the display device itself is turned on and/or when the present display mode is switched from a non-display mode to a display mode in order to save electrical power.